linux-kernel - Re: [PATCH 7/7] mm/page_alloc: Replace local

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Message-ID: <20220621092728.GE15453@techsingularity.net>
Date:   Tue, 21 Jun 2022 10:27:28 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Yu Zhao <yuzhao@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH 7/7] mm/page_alloc: Replace local_lock with normal
 spinlock

On Fri, Jun 17, 2022 at 09:57:06AM +0200, Vlastimil Babka wrote:
> On 6/16/22 23:07, Yu Zhao wrote:
> > On Thu, Jun 16, 2022 at 11:02 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> >>
> >>
> >> > @@ -3794,19 +3805,29 @@ static struct page *rmqueue_pcplist(struct zone *preferred_zone,
> >> >       struct list_head *list;
> >> >       struct page *page;
> >> >       unsigned long flags;
> >> > +     unsigned long __maybe_unused UP_flags;
> >> >
> >> > -     local_lock_irqsave(&pagesets.lock, flags);
> >> > +     /*
> >> > +      * spin_trylock_irqsave is not necessary right now as it'll only be
> >> > +      * true when contending with a remote drain. It's in place as a
> >> > +      * preparation step before converting pcp locking to spin_trylock
> >> > +      * to protect against IRQ reentry.
> >> > +      */
> >> > +     pcp_trylock_prepare(UP_flags);
> >> > +     pcp = pcp_spin_trylock_irqsave(zone->per_cpu_pageset, flags);
> >> > +     if (!pcp)
> >>
> >> Besides the missing unpin Andrew fixed, I think also this is missing
> >> pcp_trylock_finish(UP_flags); ?
> > 
> > spin_trylock only fails when trylock_finish is a NOP.
> 
> True, so it's not an active bug, but I would still add it, so it's not
> confusing and depending on non-obvious details that might later change and
> break the code.

Yes. Even though it may work, it's still wrong.

-- 
Mel Gorman
SUSE Labs